In recent years, in the field of production of medicines, gene therapy, regenerative therapy, immunotherapy and the like, it is required to culture cells, tissues, microorganisms, viruses, etc. (these are collectively referred to as “cells”) efficiently in a large amount in an artificial environment. In such cell culture, as the cell density in a culture solution (hereinafter the culture solution means one including cells and a culture medium) increases, depletion of culture medium components necessary for proliferation, and accumulation of metabolic products of the cells themselves occur, the proliferation rate decreases and the cell density reaches saturation. Therefore, when cells are cultured in a relatively large amount, culture is usually carried out while repeating subculture so that the cell density is maintained properly.
At the time of subculture, there may be cases where cells are transferred from a well plate to a flask, etc. For example, using cell culture well plates, cells are added to individual wells together with a culture medium so as to attain a proper cell density, and culture is started. After sufficiently proliferating the cells in the wells, the proliferated cells are transferred to a cell culture flask. In accordance with the progress of cell proliferation, a culture medium is added to conduct culture and also conduct subculture. When the cells are proliferated to a prescribed level, the cells are transferred to a flask or a bag having a larger capacity, and culture, supplement of a culture medium, and subculture are repeated, whereby the cells are cultured in a large amount (see paragraph [0027], etc, of Patent Document 1).
Also, a method as following is adopted when using cryopreserved cells. After thawing, culture is conducted for several days in a high density state using a well plate in order to restore the original functions of the cells (hereinafter referred to as “curing culture”), and after restoring the function of the cells to their original state, the cells are transferred to a flask and cultured (hereinafter referred to as “expansion culture”), followed by activation culture in which antibody stimulation is performed is conducted (see paragraph [0057], etc. of Patent Document 2).
In such cell culture, equipment such as a well plate or a flask has been conventionally used, but in recent years, in place of these equipment, a culture bag made of a flexible material such as a resin film has come to be used (see Patent Documents 3 and 4). Equipment such as a well plate and a flask is not suited to culture a large amount of cells. On the contrary, in the case of using a culture bag, not only the cells can be cultured in a large amount since the capacity of the bag can be increased easily, but also cell culture can be conducted in a closed system, and hence, it has a merit that risk of contamination by fungi or virus during culture can be reduced. Therefore, when the cells are cultured in a large amount on a relatively large scale (i.e. larger than the laboratory scale), a culture bag is preferably used.